Transmission mount question

Hi,

for some time now I am experiencing a clunking noise on my '90 XJS V12 when hitting pot holes etc. Searching the archives I thought the transmission mount to be the culprit. I today jacked up the car and found that the distance between the lower tie plate and the main carriage is less than a 1/4". So when I push the transmission up the tie plate almost immediately hits the carriage. I then disassembled the mount. After taking off the bottom nut and removing the tie plate I could see (looking up) the bottom end of the rubber bush and inside a spacer. Taking the spacer out I found a rounded washer. After taking off the main carrier bracket I found a spring (color coded violet and green) with plastic spring cups on both ends. The spring measured 4-4.5 inches (it is kind of crooked) unloaded including the two spring cups.
The rubber centre bush looks like new and I assume somebody replaced that in the past.

My questions:

  1. Am I correct in assuming that the bouncing space for the tie plate is too low and thus causing the clunk?

  2. Is there any part missing? I studied the parts manual and I could not find the spacer and I am not sure if part #19 is the rounded washer I found or an additional washer missing on my car.

  3. Studying the archive it seems that there his no consensus whether 1 or 2 spring cups are needed. I guess taking out 1 spring cup would lower the transmission providing more bouncing room. Correct?

  4. Does the spring data above seem to be correct or is the spring too long?

Thanks a lot for any assistance here. I know this is a topic well discussed but unfortunately I could not find the correct answers to my questions.

You should only have a rubber spring cup on one end of the spring. Having two holds the tranny too high, possibly causing your clunk.

There’s also supposed to be a washer just above the tie plate and below the spacer. It’s a funny-lookin’ thing, looks like two washers welded together. It serves as the bottom bump, the bottom end of the rubber bush should hit it before anything else hits, softening the clunk.

Could be two problems. The clunks I generally got when hitting pot holes were the front upper shock bushings which were either shot or loose.

Thanks for your responses. Greg: I am aware of the front upper shock bushings (had that before). This is definitely the transmission mount that causes the clunk now.

Kirby: This funny-looking washer is definitely missing. I am going to order it. Meanwhile, I tentatively assembled the mount again using only one spring cup. I am attaching a photo showing the resulting gap at the lower tie plate (it improved somewhat). Can somebody confirm that this gap is about correct?

P.S.: This gap would certainly increase once I insert that missing washer, I assume

On mine, the big nut on the end of that main bolt leaves no threads seen. Mine is nice and tight, and pretty much the hole is flush with the end of the bolt. Yes, it’s hard to get started, I need a floor jack, but my mount seems to work perfectly.

Mine is an 88 with the TH400.

And the missing washer is quite thick, almost the amount of threads showing on your bolt. So yes, your gap is still too tight.

Yes, the gap of the ’tongue plate’ and the mount looks a little small. I was missing both ’special washers’. The look easy to fabricate. I have the measurements below. One can take them as minimums but I wouldn’t go much larger than detailed.

Below I have diagrams of the washers.

PastedGraphic-1.png

And, here is a corrected explosion to how the barrel and washers go on the pin.

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FYI, Check archives, but i remember Kirby pointing out that 27 and 28 should be reversed.

I did reverse them to what they are now in my pictorial.

Actually, the above diagram seems to have them shown correctly: rounded-edge washer 28 goes above spacer 27. But I also recall a diagram that had them swapped.

Thanks! Waiting for the washer #21 to be delivered. Unfortunately, will take 10 days…

You are correct. Here is the diagram we were looking at last year.

WRONG:
image

Was under car today, took photo for ya!

Great photo. Anybody have an idea what the #15 spacer thickness is? Looks like I am missing those too.

It is quite thick, I’d guess 3/8" from memory.

And as i found out, they are important in making sure that rectangle plate is level. My first try, i accidentally put them on the bottom of plate, like a washer. This made plate angle down a tiny bit to where large nut is. What this did, was make post bind/rub with bushing, so transmission couldn’t easily bounce up and down. Caused a bit of vibration too.

The way yours is - missing big washer and those two spacers, at least keeps it level so it can bounce. But yours wont be able to move up as much, and will “top out” easier.

Greg, thanks for the photo. I am now convinced that with that spacer I will have the needed bouncing travel I need.

You can just stack washers to replicate the #15 spacers (there are two).

Those spacers are pretty heavy duty metal, so choose your washers wisely.

Instead of washers I am going to get flat metal and drill two holes for the studs. This will also give more surface area to stabilize the ’tongue’ rigidity. That’s why I wanted to know the spacing required that the washers provide.

Greg, is your car still on stands? Can’t you just reach under there and measure it?